1. Field of the Invention
The present invention relates to an electrically heatable windshield having an area heating element on the surface of the glass plate for the purpose of eliminating moisture condensation, melting ice or snow deposited thereon, and for defogging or anti-fogging as well as for improving durability to a current supplied to the area heating element.
2. Description of the Related Art
There have been known various types of electrically heatable windshields having a transparent conductive layer as an area heating element on the surface of the glass plate in order to prevent moisture condensation or to melt ice or snow deposited thereon.
FIG. 7 shows an example of a conventional technique. Namely, a conventional laminated glass comprises an outer glass plate 50 and an inner glass plate 52 bonded to each other through an interlayer 51 made of a material such as polyvinylbutyral, wherein a colored layer 54 for shielding is formed at the peripheral portion of the boundary surface between the outer glass plate 50 and the interlayer 51 and a transparent conductive layer 53 as an area heating element is formed at the boundary between them.
On the other hand, a bus bar 55 for supplying power is formed as a laminated layer on the colored layer 54 so that power is supplied to the transparent conductive layer 53 through the bus bar to thereby heat the glass surface.
However, since the bus bar 55 was partially extended from the colored layer 54 (i.e. an extending portion 56 was formed) as shown in FIG. 7, the breaking of a coating layer often happened at the boundary between the bus bar which was formed by printing and the transparent conductive layer 53 when a current was supplied to the bus bar.
Therefore, an improvement of the boundary has been studied. Measures to minimize the breaking of layer have been concentrated to the bus bar. For instance, there are proposals that an edge portion of the bus bar is tapered (Japanese Unexamined Utility Model Publication No. 457/1987), that the bus bar has a two-step structure at the boundary (Japanese Unexamined Utility Model Publication No. 456/1987) and that the bus bar is provided with a protective print layer (Japanese Unexamined Utility Model Publication No. 99191/1987).
Heretofore, the bus bar 55 was formed partly extended from the colored layer 54 as shown in FIG. 7. The strength of adhesion between the bus bar and the colored layer was not always sufficient. Accordingly, there was a fairly large difference of potential between them, and the breaking of layer often occurred due to the potential difference.
Techniques described in the above-mentioned publications were, therefore, proposed to improve the boundary between the bus bar 55 and the transparent conductive layer 53. Namely, these techniques provided some progress in that the boundary between the bus bar 55 and the transparent conductive layer 53 was improved, and the breaking of layer was reduced.
On the other hand, the fact that the bus bar is partially extended from the colored layer means that the color of the bus bar is visible from the outside of an automobile, this is against the standard of safety ruled by the national regulation. In the social circumstances, a demand of completely concealing the bus bar by the colored layer has been increasing.
However, in the conventional techniques (including the above-mentioned techniques even though there are some improvement), consideration that the bus bar should be entirely located on the colored layer has not been made, and it is still insufficient in the performance of the electrically heatable windshield.
Namely, as shown in a Comparative Example (FIG. 10) in which a bus bar is entirely on a colored layer in an electrically heatable windshield, when a current is supplied to the electrically heatable windshield after it has been left at a high temperature condition (80.degree. C., 28 days), the breaking of the transparent conductive layer was resulted at the contacting portion of the colored layer to the transparent conductive layer, especially at the end portion of the colored layer which is near the center of the glass plate.
The inventors of this application examined the cause of breaking and found that the following fact. There were particles of pigment having a diameter of about 1 .mu.m on the colored layer after the baking process. On the other hand, the film thickness of the transparent conductive layer was thin as 0. 1 .mu.m. Accordingly, the breaking of layer was resulted at the boundary between the bus bar and the transparent conductive layer. In particular, the breaking of layer was remarkable at the end portion of the colored layer which is near the center of the glass plate.
As the colored layer, a colored layer formed by a ceramic color print or formed by printing an organic series paint is used. The ceramic color print is composed of the glass frit, pigment and one or more additives. A bonding strength to glass mainly owes to the glass frit because it can be molten.
When the glass frit is baked, it is molten to exhibit a flat surface. Accordingly, the glass frit causes no problem. However, when the particle diameter of a component other than the glass frit is large or temperature of baking is insufficient, unevenness of the surface of the ceramic color print or the colored layer is large. Accordingly, irregularity in current conduction takes place at the colored layer, or a resistance between terminals by bonding the laminate glass becomes large. This creates the breaking of layer at the boundary between the colored layer and the transparent conductive layer.